Team studies Earth's recovery from prehistoric global warming

Apr 21, 2011 by Elizabeth K. Gardner

(PhysOrg.com) -- The Earth may be able to recover from rising carbon dioxide emissions faster than previously thought, according to evidence from a prehistoric event analyzed by a Purdue University-led team.

When faced with high levels of atmospheric carbon dioxide and rising temperatures 56 million years ago, the Earth increased its ability to pull carbon from the air. This led to a recovery that was quicker than anticipated by many models of the carbon cycle - though still on the order of tens of thousands of years, said Gabriel Bowen, the associate professor of earth and atmospheric sciences who led the study.

"We found that more than half of the added carbon dioxide was pulled from the atmosphere within 30,000 to 40,000 years, which is one-third of the time span previously thought," said Bowen, who also is a member of the Purdue Climate Change Research Center. "We still don't know exactly where this carbon went, but the evidence suggests it was a much more dynamic response than traditional models represent."

Bowen worked with James Zachos, a professor of earth and planetary sciences at the University of California, Santa Cruz, to study the end of the Palaeocene-Eocene Thermal Maximum, an approximately 170,000-year-long period of global warming that has many features in common with the world's current situation, he said.

"During this prehistoric event billions of tons of carbon was released into the ocean, atmosphere and biosphere, causing warming of about 5 degrees Celsius," Bowen said. "This is a good analog for the carbon being released from fossil fuels today."

Scientists have known of this prehistoric event for 20 years, but how the system recovered and returned to normal atmospheric levels has remained a mystery.

Bowen and Zachos examined samples of marine and terrestrial sediments deposited throughout the event. The team measured the levels of two different types of carbon atoms, the isotopes carbon-12 and carbon-13. The ratio of these isotopes changes as carbon dioxide is drawn from or added to the atmosphere during the growth or decay of organic matter.

Plants prefer carbon-12 during photosynthesis, and when they accelerate their uptake of carbon dioxide it shifts the carbon isotope ratio in the atmosphere. This shift is then reflected in the carbon isotopes present in rock minerals formed by reactions involving atmospheric carbon dioxide, Bowen said.

"The rate of the carbon isotope change in rock minerals tells us how rapidly the carbon dioxide was pulled from the atmosphere," he said. "We can see the fluxes of carbon dioxide in to and out of the atmosphere. At the beginning of the event we see a shift indicating that a lot of organic-derived carbon dioxide had been added to the atmosphere, and at the end of the event we see a shift indicating that a lot of carbon dioxide was taken up as organic carbon and thus removed from the atmosphere."

A paper detailing the team's National Science Foundation-funded work was published in Nature Geoscience.

It had been thought that a slow and fairly constant recovery began soon after excess carbon entered the atmosphere and that the weathering of rocks, called silicate weathering, dictated the timing of the response.

Atmospheric carbon dioxide that reacts with silicon-based minerals in rocks is pulled from the air and captured in the end product of the reaction. This mechanism has a fairly direct correlation with the amount of carbon dioxide in the atmosphere and occurs relatively slowly, Bowen said.

The changes Bowen and Zachos found during the Palaeocene-Eocene Thermal Maximum went beyond the effects expected from silicate weathering, he said.

"It seems there was actually a long period of higher levels of atmospheric carbon dioxide followed by a short and rapid recovery to normal levels," he said. "During the recovery, the rate at which carbon was pulled from the atmosphere was an order of magnitude greater than the slow drawdown of carbon expected from silicate weathering alone."

A rapid growth of the biosphere, with a spread of forests, plants and carbon-rich soils to take in the excess carbon dioxide, could explain the quick recovery, Bowen said.

"Expansion of the biosphere is one plausible mechanism for the rapid recovery, but in order to take up this much carbon in forests and soils there must have first been a massive depletion of these carbon stocks," he said. "We don't currently know where all the carbon that caused this event came from, and our results suggest the troubling possibility that widespread decay or burning of large parts of the continental biosphere may have been involved."

Release from a different source, such as volcanoes or sea floor sediments, may have started the event, he said.

"The release of carbon from the biosphere may have occurred as a positive feedback to the warming," Bowen said. "The forests may have dried out, which can lead to die off and forest fires. If we take the Earth's future climate to a place where that feedback starts to happen we could see accelerated rates of climate change."

The team continues to work on new models of the carbon cycle and is also investigating changes in the water cycle during the Palaeocene-Eocene Thermal Maximum.

"We need to figure out where the carbon went all those years ago to know where it could go in the future," he said. "These findings show that the Earth's response is much more dynamic than we thought and highlight the importance of feedback loops in the carbon cycle."

More information: Rapid Carbon Sequestration at the Termination of the Palaeocene-Eocene Thermal Maximum, by Gabriel J. Bowen and James C. Zachos

AbstractThe Palaeocene-Eocene Thermal Maximum (PETM), an approximately 170,000-year-long period of global warming about 56 million years ago, has been attributed to the release of thousands of petagrams of reduced carbon into the ocean, atmosphere and biosphere. However, the fate of this excess carbon at the end of the event is poorly constrained: drawdown of atmospheric carbon dioxide has been attributed to an increase in the weathering of silicates or to increased rates of organic carbon burial. Here we develop constraints on the rate of carbon drawdown based on rates of carbon isotope change in well-dated marine and terrestrial sediments spanning the event. We find that the rate of recovery is an order of magnitude more rapid than that expected for carbon drawdown by silicate weathering alone. Unless existing estimates of carbon stocks and cycling during this time are widely inaccurate, our results imply that more than 2,000 Pg of carbon were sequestered as organic carbon over the 30,000-40,000 years at the end of the PETM. We suggest that the accelerated sequestration of organic carbon could reflect the regrowth of carbon stocks in the biosphere or shallow lithosphere that were released at the onset of the event.

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The Snowball Earth events of 800 to 560 million years ago each time resulted in far higher carbon dioxide levels but plants require Carbon dioxide for their various parts and sucked it out of the atmosphere and tilted the climat back into a frozen period. Anyone who has ever worked with plants in environments with enriched carbon dioxide would not be asking this question but quantifying how much. You can have a very good feel for how much carbon dioxide is in the atmosphere by the ration of grass lands that require less carbon dioxide, to forest lands. We can be certain that the forest lands expanded in the period in question.

We humans plant plants. The speed of changes in human managed forests is difficult to quantify but very much higher than natural ecosystems without humans present. The rate may be a hundred times and the temperature rise was only 5 degree C anyway. So you have to sell your beach front property in this century. The rising evaporation of the oceans will increase the area of arable land because of increased rainfall.

We learned in Cub Scouts eons ago that vegetation needs carbon dioxide to survive and grow and that there is a symbiotic relationship between plants and animals. Seems quite simple that Avitar is right; plant plants and trees.

The rising evaporation of the oceans will increase the area of arable land because of increased rainfall.

No it won't. Evaporation rates over land will increase even more than over oceans. Inner-continental lands far from shores will dry out and desertify. Whereas lands closer to shores that currently experience adequate rain will indeed experience much more rainfall, which is liable to turn them into swamps, wash away topsoil, and pose infrastructure challenges. The tropics (where most human population is concentrated today) will lose productive lands, while the arctic regions (where relatively few people live) will gain arable land -- but also suffer expanded swamps and liquefying permafrost.

On the whole (adding up gains and losses across the globe), current projections say arable land will decrease as a result of global warming/climate change. Also, warmer oceans are less productive -- so big declines in seafood availability (in addition to stress from overfishing.)

We learned in Cub Scouts eons ago that vegetation needs carbon dioxide to survive and grow

It needs a lot more than carbon dioxide. It needs water, and it needs nitrates and phosphates. Raising CO2 in the atmosphere won't increase availability of other essential nutrients in the topsoil, and this scarcity will be a limiting factor; therefore the effect of carbon fertilization across the globe will be rather underwhelming on average.

The truth about the carbon cycle is told at last. The only thing we have to fear is fear itself. This exposes the whole "carbon footprint" thing as nothing more than a tool to generate revenue and stop progress. People need to stop living in fear and realise that the carbon released in the air today comes back as food later for all living things. It is called the great circle of life.

On the whole (adding up gains and losses across the globe), current projections say arable land will decrease as a result of global warming/climate change. Also, warmer oceans are less productive -- so big declines in seafood availability (in addition to stress from overfishing.

I agree with this. Combined with ocean acidification from excess CO2, we should wake up now to realize that we need to stop and reverse this CO2 (and other) pollution now because every day it will become more and more costly in terms of effort needed and harms done.

The truth about the carbon cycle is told at last. The only thing we have to fear is fear itself. This exposes the whole "carbon footprint" thing as nothing more than a tool to generate revenue and stop progress. People need to stop living in fear and realise that the carbon released in the air today comes back as food later for all living things. It is called the great circle of life.

You're an idiot, the so called carbon cycle is based on naturally occurring processes(over tens of thousands of years) not multiple industrialized nations spewing CO2 and other noxious gasses into the atomsphere(in what, the last 150 years?). Couple that with heavy deforestation and you've got a recipe for disaster.

I'm not saying the earth won't ever recover, but mankind is going to pay a heavy price before it does.

Congratulations to the Purdue team for reminding us of the record of variations in Earth's climate long before the industrial revolution!

Al Gore, the UN's IPCC, and the army of consensus scientists promoting CO2-induced global warming ignored the historical record of variations in Earth's climate and claimed that CO2 must be the cause of global warming.

Dead zones in the ocean are excellent carbon sinks. There's no O2 near or at the bottom, so dead algae/plankton just pile up to form a nasty slime. If that gets capped with mud which becomes shale, it eventually becomes a petroleum deposit. You want more oil? Just kill the oceans for a million years or so.

Forest fires in Indonesia in 1997 were estimated to have released between 1 and 2.5 gigatonnes of CO2 into the atmosphere. Perhaps we are beginning to experience these positive feedback events right now.When the methane from the permafrost, and the methane, stored as frozen hydrate in sediments on the ocean floor, are released into the atmosphere due to warming atmosphere and the oceans, it seems likely that these feedback events will run amok. Forget about CO2 because when all that methane is released global warming will most likely spiral out of control..

CO2 only seems to be be the catalyst which provides the first spark. Then when the methane is released global warming will spiral out of control. Sadly I can't see a solution for this progression. All is in place now.

Whatever you believe will happen to our race and planet, act accordingly! Invest and plan for what your senses and reason have convinced you of.

There is no need to argue - If you believe we are doomed and the world will burn up from our CO2 emissions, live like it. Move to the caves, sell your automobile, dress in hemp or burlap and ring your doom bell on the corners of the cities, if you feel activist.

Others, who don't believe our human life processes are particularly fatal for this earth - You should continue to consume and expect the harvest to remain full and abundant. If the world does not burn up, those who positioned themselves correctly will profit the most from their investments in lifestyle and will take us forward.

These two ideologies running in parallel in our society is an advantage. If the world does indeed burn up, then we will carry on with those who foresaw the doom and lived in a manner that allowed them to survive.

admittedly, I probably took that one too far with mr Manuel, but I'm getting pretty sick of his comments.

For the record, I did research his papers and positions - It appears that he did make a couple of pertinent observations, then proceeded to immediately extrapolate them to extremes outside of his knowledge area, throwing out logical explanations by people with the right credentials, sometimes by people who independantly made the same observations as him.

Basically, go ahead and look up his papers at the university of missouri technology. He has one observation about neutron behavior, takes it to an extreme, and has virtually no theoretical framework to get from "A" (A magnetic nucleon, neutrons have charged constituents, but a net 0 charge) don't behave exactly as a completely neutral nucleon should, to "Z" that neutrons power the sun instead of fusion. I'm sure most of us are tired of him.